CATV distribution systems typically include active devices and passive devices that are physically connected via main line distribution cables that have a metallic center conductor provided within a metallic sheath and surrounded by an insulator. Examples of the active devices typically included in a main line distribution cable of a CATV distribution system include amplifiers, line extenders, and status monitoring equipment. Examples of passive devices included in a CATV distribution system include directional taps that feed the television signal (RF signal) carried on the main line distribution cable to a drop cable that carries the signal to individual subscriber residences.
In the past, the only devices that required power were those coupled to the main line distribution cables of the distribution network, such as the active devices described above. Those devices were typically powered with a 60 volt RMS, 60 Hertz signal that was carried on the main line distribution cable along with the other television (RF) signals. The power supply units that provide this 60 volt RMS, 60 Hertz signal waveform to the main line distribution cable are generally backed up by internal batteries that allow the distribution system to continue to operate in the event of a power failure.
It is becoming more common to provide an active device at the end of the drop cable, e.g., attached to an exterior portion of a subscriber residence, to provide an alternative access to local and long distance telephone service that would normally be supplied by a local telephone service system operator. In order for the CATV operator to supply a service that is similar to that of the incumbent telephone service, it is desired to power the active device, also known as a Network Interface Unit (NIU), located in or on each subscriber's residence with the same voltage sources that are used to power the main line distribution system. This will allow the NIU to continue to operate even in the event of a power failure.
The voltage supplied to the main line distribution cable is subject to a series of voltage drops associated with each active and passive device carried by the main line distribution cable, and by the series resistance of the main line cable itself, connected between the active and passive devices, which ultimately limits the number of devices that can be powered effectively from a single power supply unit. Since each active device within the distribution system consumes a certain amount of power, the voltage available for subsequent active devices drops due to the voltage drop associated with each successive active device. Consequently, auxiliary power supply units need to be positioned at predetermined intervals along the main line distribution cables to provide sufficient voltage for successive downstream active devices.
In view of the extra loading that this additional powering requirement imposes on the overall distribution network, the designers of the powering components and active distribution equipment have elected to increase the voltage carried on the main line distribution cable to compensate for the increased power requirements. In many cases the providers for CATV systems elected to increase the voltage level carried on the main line distribution cables from 60 volts RMS to 90 volts RMS. Increasing the voltage increases the amount of operational current along the line, which allows the number and frequency of auxiliary power supply units to be reduced. The decision to increase the voltage to 90 volts RMS was also driven by the desire to reduce the number and/or frequency of auxiliary power supply units provided intermittently along the main line distribution cables.
This new voltage level, however, exceeds the limits for drop cables set by the National Electric Code (NEC) and the energy limitations stated in Section 820-4 of Community Antenna Television and Radio Distribution Systems, Article 820. Accordingly, cable operators providing a 90 volt RMS power signal are subject to the regulations imposed under Article 830 of the NEC, which sets forth the standards for cable wiring having a power level above 60 volts RMS. For example, NEC 830 requires burying cable lines carrying more than 60 volts RMS, or requires an additional safeguard device such as a voltage monitor with a power cut-off feature, to prevent injury and equipment damage in the event that the integrity of the drop cable is somehow breached. The more stringent standards of NEC 830 result in a significant increase in hardware cost.
To meet the requirements of NEC 830, many providers in the CATV industry currently employ a device called a “Drop Check” such as the device described in U.S. Pat. No. 5,793,590 to Vokey et al. The '590 patent describes a two part device used to monitor the integrity of both the center conductor and the outer conductor of the drop cable that operates in a manner similar to a GFI (Ground Fault Interrupt) device in standard residential wiring systems. If the pole mounted portion of the device, which is directly connected to the output conductor of the directional tap, does not sense the second portion of its circuit that resides at the other end of the drop cable within the NIU enclosure (at the NIU input), the 90 volt AC signal is automatically interrupted at the pole mounted device. Any interference with the drop cable line, such as a break or interruption of the structural integrity of the outer conductor, for example, will trip the circuit so that the power is cut from the drop cable at the pole. In that manner, unexpected contact with the damaged drop cable is unlikely to result in injuries such as burns, fires or electrocution, caused by contact with the current carried by the drop cable.
Since the “Drop Check” device has been implemented in the field, however, CATV providers have recognized several significant problems. For example, the structure of the device itself includes inherent power inefficiencies. Since the device only conducts over ¼ of the signal waveform, its efficiency is effectively reduced. In addition, the reliability of the device is notoriously low, as well, and a high mortality (failure) rate in the field has been reported. Still further, the devices are quite costly, and there are undesirable costs associated with implementing and maintaining the multiple component parts of the device.
In view of the foregoing, it would be desirable to provide a highly reliable, highly efficient device that can be installed in communication with main line distribution cables, preferably at the head-end of the drop cables, such that a CATV provider can power successive active units (e.g., NIUs) with the least possible number of auxiliary power supply units, and at the same time reduce the power actually fed to the drop cables so as to remain within the less burdensome requirements of NEC 820.
It would be more desirable to provide a highly reliable, highly efficient and highly cost effective voltage limiter that can be used in connection with a directional tap to limit the voltage carried on a drop cable extending from the main line distribution cable to a lower voltage so as to comply with the applicable safety regulations while ensuring that sufficient power is available on the main line distribution cables to power a higher number of downstream active devices.